The invention relates to wind turbines, and in particular, to wind turbines with articulated blades.
With the diminishing global supply of fossil fuel, renewable sources of energy such as power generated from wind turbines is gaining an increasing share of the electric power market. Thus, generating electrical power more efficiently with wind turbines has become increasingly important to lowering the overall cost of electricity.
It is known that the conversion efficiency of wind generated kinetic energy to electric power is proportional to the square of the length of the blades of a wind turbine. Thus, increasing the length of the blades allows the wind turbine to produce power more efficiently. Lengthy blades currently face three major challenges that have negatively impacted their implementation on wind turbines. First, lengthy blades (current blades can have a length of up to about 50 meters, future blade lengths of up to 80 meters have been contemplated) can be difficult to transport and install. Special large trucks are needed to transport the blades from the manufacturing site along routes to an on-shore implementation site. The routes must be carefully planned so as to accommodate the large trucks and cargo. Specially designed tall cranes may also be required to hoist the blades to the wind turbine tower for installation. In view of these and other challenges, transportation/installation cost can be as high as one third of the manufacturing cost of the blades.
Lengthy blades also have high velocity gradients from the root of the blade to the tip of the blade. High velocity gradients complicate the engineering and fabrication of the wind turbine and blades. For instance, blades (especially those with high velocity gradients) must be designed with a degree of twist (commonly called pitch) from root to tip so as to optimize performance of the blade for a most common wind velocity expected to be experienced at the installation site. Adding twist to blades adds to the complexity of their manufacturing and their cost. Once installed, the twist of the blade commonly remains fixed such that if the wind blows with an unpredicted (non-optimized) speed the conversion efficiency of the wind turbine is reduced.
Additionally, wind turbines utilizing lengthy blades must be shut down during periods of high wind so as to avoid damaging wind turbine parts such as the blades, bearings, gears and support tower. In contrast, wind turbines that utilize shorter blades can operate in higher wind conditions without having to be shut down.
Lengthy blades commonly have a low bending stiffness which can negatively impact the wind turbine conversion efficiency and wind turbine operating life through blade deformation and changes to the natural frequency of the blade. Measures such as extra material reinforcement may be required to reduce blade deformation and changes in the natural frequency of the blade. However, these measures increase the manufacturing cost and weight of the blade.